1. Field of the Invention
The present invention relates to a heating apparatus having at least two heating bodies and an image forming apparatus using such a heating apparatus as a thermal fixing apparatus for fixing an image.
2. Related Background Art
A conventional image forming apparatus using an electro-photographic process will now be explained.
A thermal fixing apparatus in the image forming apparatus serves to fix an unfixed image (toner image), which was formed on a recording paper by image forming means such as an electro-photographic process, on the recording paper, and a thermal fixing apparatus of heat roller type utilizing a halogen heater as a heat source and a thermal fixing apparatus of film heating type utilizing a heater in which a ceramic surface thereof generates heat (referred to as “heater” hereinafter) as a heat source are known.
FIG. 7A is a schematic sectional view showing a fixing nip portion of an example of a fixing apparatus of film heating type utilizing a heating member having two heating bodies as a heat source. FIG. 7B is a partially fragmental schematic plan view showing a front side of the heating member and FIG. 7C is a schematic plan view showing a back side of the heating member.
In FIG. 7A, a heating member as a heater 1 includes two heating bodies in this example. The heater 1 is held and fixed by a heat-resistive rigid support 2. A heat-resistive film (referred to as “fixing film” hereinafter) 3 as a shifting member is closely contacted with the heater 1 by a pressurizing roller 4 as a pressurizing member and is slidingly shifted so that a recording material S as a heated material onto which an image is to be fixed is introduced between the fixing film 3 and the pressurizing roller 4 of an abutment nip portion (referred to as “fixing nip portion” hereinafter) N defined between the heater 1 and the pressurizing roller 4 with the interposition of the fixing film 3, with the result that, by conveying the recording material together with the fixing film 3 through the fixing nip portion N, heat from the heater is applied to the recording material S via the fixing film 3, thereby thermally fixing an unfixed toner image T on the recording material S onto a surface of the recording material S. The recording material S passed through the fixing nip portion N is separated from a surface of the fixing film 3 and then is conveyed.
In the heater 1, there is provided a ceramic insulating substrate (referred to as “heater substrate”) 1a which is made of Al2O3 or AlN and which is an elongated heat-resistive and insulating and good heat-conductive member having a longitudinal direction perpendicular to a conveying direction a of the fixing film 3 or the recording material S as the heated material.
There are further provided a first or long heating body 1b and a second or short heating body 1c which are formed on a front side of the insulating substrate 1a and arranged side by side along the longitudinal direction of the substrate. The two heating bodies 1b and 1c are formed by patterning and baking by means of screen printing and the like using electrically heat generating resistance paste such as silver palladium (Ag/Pd), Ta2N or the like.
A length of the first heating body 1b corresponds to a width of a fed sheet of a recording material having a maximum size which can be fed through the apparatus (width of large size fed sheet). A length of the second heating body 1c corresponds to a width of a small size recording material which can be fed (width of small size fed sheet). Incidentally, in this example, conveyance of the recording material S is based on center reference.
There are further provided load-dispatching wiring patterns 1f and 1g for the first heating body 1b, which are formed on the surface of the insulating substrate to be electrically connected to both ends of the first heating body 1b and load-dispatching wiring patterns 1h and 1i for the second heating body 1c, which are formed on the surface of the insulating substrate to be electrically connected to both ends of the second heating body 1c. The load-dispatching wiring patterns 1f, 1g, 1h and 1i are formed by patterning and baking by means of screen printing and the like using conductive paste such as silver (Ag).
A glass coating baking layer 1d serves to protect the surface of the heater and to ensure insulation and is formed to partially cover the first heating body 1b and the second heating body 1c on an area of the heater surface corresponding to the width of large size fed sheet and the load-dispatching wiring patterns 1f, 1g, 1h and 1i. 
A temperature detecting element 1e such as a thermistor for detecting a temperature of the heater is contacted with a longitudinal central portion on the back surface of the heater, i.e. back surface of the insulating substrate.
The heater 1 is held and fixed by the support 2 in such a manner that the front surface of the heater on which the heating bodies 1b, 1c and the like is exposed downwardly. Load-dispatching connectors (not shown) are connected to both ends of the heater 1 so that an electrical power is supplied to the first heating body 1b from a heater driving circuit through the load-dispatching connectors and the load-dispatching wiring patterns 1f and 1g, thereby heating the first heating body 1b. Further, an electrical power is supplied to the second heating body 1c through the load-dispatching connectors and the load-dispatching wiring patterns 1h and 1i, thereby heating the second heating body 1c. The temperature of the entire heater 1 can quickly be increased by heating the first or second or both heating bodies 1b and 1c. 
An example of a general heater driving circuit is shown in FIG. 8. As mentioned above, the heater 1 includes two heating bodies 1b and 1c and is connected to a commercial AC power supply 201 via switching control elements 204 and 219 such as Triacs so that an electrical power is supplied to the respective heating bodies 1b and 1c from the AC power supply 201.
The temperature of the heater 1 is detected by the thermistor temperature detecting element 1e disposed on the rear surface of the heater and an engine controller 212 as a control circuit performs phase control by controlling ON/OFF of the switching control elements 204 and 219 thereby to turning ON or OFF the supplying of the power to the heater 1 so that the temperature of the thermal fixing apparatus is controlled to be maintained to a target constant temperature. The temperature detected by the thermistor temperature detecting element 1e is detected as divided voltage of a resistor 215 and the temperature detecting element 1e and is A/D-inputted to the engine controller 212 as a TH signal.
Regarding the two heating bodies 1b and 1c of the heater 1, voltages supplied thereto are appropriately controlled respectively in accordance with the size of the paper on which the printing is performed, and the printing is performed while switching the lighting ratio of the heating body. In this case, the respective heating bodies 1b and 1c are phase-driven independently in accordance with the voltages supplied thereto.
A safety device 213 is also provided so that, if the heater detection temperature of the thermistor temperature detecting element 1e becomes above an allowable value, the supplying of the electrical power to the heater 1 is stopped immediately by activating the safety device 213.
However, in the above-mentioned conventional example, since the respective heating bodies 1b and 1c are phase-controlled independently, regarding electrical current flowing in total, as shown in FIG. 9, a wave form of the current is suddenly changed on the way of an energizing period for each half wave, with the result that there arises a problem that a bad influence is affected upon equipments connected to the common commercial power supply, such that a frequency component which increases terminal noise voltage is generated.